I am not from RME, my opinion on this is:
ADI-2 Pro has latest converter technology and is aimed to be reference class.
Very attractive to be used to drive high quality monitors and headphones.
Maybe the best is - as usual - to get both and then to benchmark them inside of your toolchain.
We have not your room, your ears, your tools, your monitors.
My question to you would be, where your focus is and whether you need
- high quality D/A conversion for headphones / monitors or
- more channels anyway where you would need the ADI-8 QS
This you can only answer to yourself.
It maybe also depends on your setup .. where you need the channels.
the ADU-8 QS offers MADI, thus also a flexibility in terms of where to place the unit.
Best is surely to get both devices as you might have use cases for both of them.
- getting highest quality for Headphones and Speakers and uses cases like SRC, USB DAC, mobile headphone amp
- being able to have more I/O channels in high quality ...
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13